The removal of sulfur compounds from sulfur-containing gas streams is an important environmental process. For this reason, various emission requirements limit the amount of sulfur compounds which can be emitted into the atmosphere since sulfur compounds in gaseous emissions can result in the pollution of the atmosphere which can produce undesirable results such as acid rain and the like. Furthermore, in many cases when a gas stream containing sulfur contaminants is processed, the sulfur compounds can poison sulfur sensitive catalysts, corrode equipment, or have other adverse effects.
One technique for the desulfurization of sulfur-containing gas streams involves heating the gas stream with particulate materials that absorb objectional sulfur compounds. These sulfur absorbing materials or "sorbents" are typically high surface area, highly porous materials capable of removing sufficient quantities of sulfur compounds so that the treated gas streams exhibit very low sulfur content and thus can meet emission requirements for sulfur compounds. One conventional class of sorbents for absorbing sulfur compounds are active metal oxide sorbents which include supported and unsupported active metal oxides derived from the calcination of various individual and mixed active metal oxides.
For example, U.S. Pat. No. 4,088,736 to Courty et al. proposes a zinc oxide sorbent which is supported on silica and/or alumina. Other sorbents for removing sulfur compounds that are derived from the calcination of active metal oxides include the sorbents described in U.S. Pat. No. 4,769,045 to Grindley which proposes a zinc ferrite sorbent prepared from the mixing and calcining equimolar amounts of zinc oxide and iron oxide. U.S. Pat. Nos. 4,312,820 and 4,725,415, both assigned to Phillips Petroleum Company, propose the use of zinc titanate sorbents formed from the mixing and calcination of zinc oxide and titanium dioxide. U.S. Pat. No. 5,254,516 to Gupta et al. also discloses zinc titanate sorbents. Additionally, U.S. Pat. No. 4,977,123 to Flytzani-Stephanopolous et al., proposes a method of making mixed active metal oxide sorbents prepared using calcined powders of oxides of various metals such as for example, copper, iron, aluminum, zinc, titanium, and mixtures thereof to form the sorbent material.
One desirable application for active metal oxide sorbents is the removal of sulfur compounds from fuel gas streams. Specifically, active metal oxide sorbents are particularly desirable for use in the desulfurization of coal gas streams that are used as fuel for power generation systems. These systems convert chemical energy stored in coal to electricity by first generating fuel gas via coal gasification, and then oxidizing the hot gas in either a turbine or a fuel cell. This approach, however, is complicated by the presence of sulfur in coal, which is converted to reduced sulfur species such as H.sub.2 S, COS, and CS.sub.2 during gasification. Subsequently, during combustion of the fuel gas, the H.sub.2 S oxidizes to SO.sub.2 which can cause the formation of acid rain if discharged into the atmosphere. In addition to environmental concerns, high concentrations of H.sub.2 S can be corrosive to energy producing equipment and can adversely affect the performance of molten carbonate fuel cells due to sulfur poisoning of electrodes.
In the conventional method of removing sulfur compounds such as H.sub.2 S, COS and CS.sub.2 from coal gas streams, a hot gas stream is fed from the gasifier to an absorber at a temperature of between about 800.degree. F. and 1000.degree. F. The absorber is typically a fixed-bed, fluidized bed, or moving bed reactor containing a particulate supported or unsupported active metal oxide sorbent. The particles containing the active metal oxide sorbent are intimately contacted with the hot gas stream entering the reactor resulting in absorption of the sulfur compounds by the active metal oxide sorbents, i.e., reaction of the active metal oxide and sulfur compound to form a metal sulfide (a sulfided sorbent) and typically either water or carbon dioxide.
In the conventional desulfurization process, the metal sulfides derived from the active metal oxide sorbents are recovered from the desulfurization process and transported to an adiabatic bed reactor for regeneration. In the regenerator, the fluidizable particles containing the sulfided sorbents are regenerated in an oxygen-containing gas stream such as an oxygen-enriched, a diluted, or an undiluted atmospheric air stream. The sulfided sorbent reacts exothermically with oxygen and is regenerated to the active metal oxide based sorbent and forms sulfur dioxide as a by-product.
In the regenerator, the temperature necessary to effectively initiate the regeneration reaction is typically in excess of about 1000.degree. F. Although part of the heat necessary to initiate the reaction is supplied by the sulfided sorbent which, as described above, is heated to a typical temperature of between about 800.degree. F. and 1000.degree. F. during the desulfurization process, the heat carried by the sorbent is generally below the temperature necessary for start up of the regeneration process. For example, conventional zinc oxide and zinc titanate sorbents typically require a regeneration temperature in the range of between about 1150.degree. F. and 1400.degree. F. Although various modifications have been proposed to provide sorbents having somewhat lower initiation temperatures for regeneration thereof, such modifications can lead to various other complications. Thus, for example, U.S. Pat. No. 5,439,867 to Khare et al., and assigned to Phillips Petroleum, describes a zinc oxide based sorbent containing a nickel oxide or nickel nitrate promoter wherein the sorbent is regenerated at temperatures of about 1100.degree. F. and 1200.degree. F. However, the nickel promoter tends to increase disposal problems associated with the spent sorbent, and during use of the sorbent for sulfur removal from fuel gas streams, also tends to catalyze the formation of methane from carbon monoxide and hydrogen, which depletes the energy in the desulfurized fuel gas.
In any case, because the regeneration reactor is adiabatic, start up of the regeneration reaction is traditionally initiated by raising the temperature of the oxygen-containing gas fed to the regenerator to a temperature above the initiation temperature of the regeneration reaction. However, heating of the oxygen-containing gas stream requires increased capital investment in various heating and associated apparatus, and is also energy intensive and thus substantially increases the costs associated with sulfur removal from fuel gas streams. Moreover, in some cases, such as transport reactor-based processes, residence time of the sorbent in the regenerator may not be sufficient to provide the temperature rise needed to achieve regeneration of sulfided sorbent.